Since its conception, transcranial magnetic stimulation (TMS) has become an important tool in neurophysiology and biological psychiatry. The application of repetitive stimulation has been shown to produce changes in brain activity and it is increasingly being applied in therapeutic applications such as major depressive disorder, schizophrenia and autism.
TMS may be applied to various regions of the brain including the motor cortex, and the prefrontal cortex. Commonly, TMS is applied non-invasively using a coil, such as a ‘figure-of-8’ coil, which is positioned externally to the patient and held adjacent the target cortical region. A large transient current is delivered to the coil, generating one or more magnetic pulses that induce ionic current flow in the cortex that can depolarize (activate) neurons.
In addition to the intensity of the magnetic stimulation, the degree of elicited neuron activity is affected by the coil orientation relative to the scalp. This is a direct result of neurons being preferentially stimulated by ionic current travelling in a particular direction, the direction of ionic current flow being altered by altering the coil orientation. The optimal orientation for stimulating the motor cortex, for example, is typically 45° to a parasagittal plane of the patient.
To ensure that desired therapeutic outcomes can be realised, it is important to determine a threshold at which the TMS provides neuron depolarization and thus a therapeutically relevant change in neuron activity. In the case of TMS applied to the motor cortex, for example, a stimulation intensity required to activate a certain muscle, e.g., a thumb, is known as the resting motor threshold (rMT).
However, therapeutic treatments using TMS typically target cortical brain regions that are outside of the motor cortex such as the prefrontal cortex (PFC), in which regions TMS elicits no readily observable indication that a threshold stimulation level has been reached. Overall, there has been no straightforward method for determining the threshold stimulation level or optimal coil orientation (appropriate “TMS parameters”) at the PFC or other brain regions outside of the motor cortex at which neuron depolarization takes place.
Traditionally, it has been assumed the TMS parameters that are appropriate for the motor cortex will also be appropriate at the other target cortical regions. However, this is not necessarily the case due to differences in the scalp to cortex distance at different regions of the scalp and other factors such as variation in direction of nerve fibres. The assumption may therefore lead to reduced TMS treatment efficacy for certain patients. It follows that, by determining TMS parameters specific to a target cortical region, treatment efficacy can be improved.
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